A conventional near infrared region sensor card which is used for identifying near infrared light is composed of laminated plastic sheets and a photoluminescent material dispersed as fine powders between the plastic sheets.
On receiving near infrared light, the sensor card emits visible light and shows the form of the invisible near infrared rays and the intensity of the incident light as it is. Thus, the sensor card is effective for the detection of the position of near infrared rays emitted from various kinds of lasers and LED (light emitting diode) and the identification of patterns of such near infrared rays. Also, since a photoluminescent material is used for converting near infrared rays into visible light, the sensor card has an advantage that it can be repeatedly used.
However, since the sensor utilizes the luminous phenomena of a photoluminescent material caused by the transition from an excited state thereof raised by absorbing a light energy receiving from outside, there are various inconveniences. That is, in an ordinary room light, it is required to continue the irradiation for a time as long about one minute for bringing the photoluminescent material into the excited state, and also since the luminescence is vanished with the stop of the light irradiation, photographing is required for storing the luminous pattern by the irradiated light and hence without photographing, it is impossible to identify the state of the irradiated light in detail or compare the state of the irradiation light with a state of other irradiation light.
Furthermore, a near infrared region sensor card using a photoluminescent material is expensive and an inexpensive and simply disposable near infrared region sensor card has not yet been developed.
As described above, in the conventional method of identifying near infrared light such as semiconductor laser light, there are disadvantages that it is difficult to obtain records for comparing one irradiated pattern with other irradiated pattern, it is impossible to store the records easily (or without need of photographing) for a long period of time, and also the materials being used for the method are expensive.
In particular, in the case of focusing a laser beam, it is really necessary to easily obtain the recorded image of the focused laser beam, which can safely and simply detect the shape, intensity, etc., of the focused laser spot and can compare it with other recorded images later, but such an easy means has not yet been found at present. Also, when a laser device emits plural laser beams, it is difficult to exactly know the number, shapes, and intensities of these laser beams by conventional methods.
Also, in the case of focusing plural laser beams, it is impossible to accurately determine whether the optical systems are matched well or not.